IEEE 802.11 is a set of media access control (MAC) and physical layer (PHY) specification for implementing wireless local area network (WLAN) communication, in the unlicensed (2.4, 3.6, 5, and 60 GHz) frequency bands. The standards and amendments provide the basis for wireless network products using the IEEE 802.11 frequency bands. IEEE 802.11 plays an important role in the growing application of Indoor/Outdoor Location. The key applicable technology is that of ranging using time-of-flight (TOF) ranging measurements defined in IEEE 802.11v. Once the distance between devices is measured, the information can be used to determine device location.
In IEEE 802.11-REVmc, Fine Timing Measurement (FTM) protocol is proposed for Indoor Location. Based on FTM, an initiating station exchanges FTM frames with a responding station to measure the time-of-flight (TOF) or the Round Trip Delay (RTD/2). The initiating station then computes its range to the responding station after receiving timing measurements (i.e., timestamps corresponding to the departure time and arrival time of the FTM frames) from the responding station. To calculate a station position, the station performs ranging measurements with multiple access points (APs) via FTM frame exchange and obtains AP's positions. FTM positioning requires the initiating station to exchange FTM frames with multiple responding APs for TOF measurements in order to determine its absolute location. For 3D positioning, the station needs to exchange FTM frames with four APs in order to determine its absolute location.
FIG. 1 (Prior Art) illustrates conceptually an FTM positioning procedure. As illustrated in FIG. 1, a typical FTM positioning procedure starts with a setup phase, followed with a measurement phase, and ended with a tear down phase. During the setup phase, the initiating station STA sends an FTM request or FTM requests to each of the responding APs, AP1-AP4, and wait for acknowledgments (ACKs) from AP1-AP4. In communicating with each responding AP, the initiating station STA might be required to switch to the operating channel of the responding AP if the station's current channel is different from the operating channels of the AP. The FTM request contains FTM related parameters including the start time and the duration of the subsequent FTM frame exchange. After the setup phase, the STA starts the measurement phase with each AP. During each measurement session, the STA exchanges FTM frames and obtains measurement results for TOF with each AP and thereby computing its distance to the corresponding AP. A dialog token carried within the FTM frame is used as an identification of the corresponding timestamp measurements at the AP and the STA. Timestamps and dialog token, which identifies the FTM (and ACK) frames in which timestamps are measured, are transferred together for use in range calculation. Again, channel switching between each measurement session may be needed if the wireless operating channels of the APs are different. After four measurement sessions with AP1-AP4 are completed, the STA is then able to determine its absolute location given the APs' positions. Finally, the STA and the APs go through a tear down phase to end the FTM positioning procedure if the tear town has not already been specified during the set up phase.
The FTM protocol suffers from a list of drawbacks. First, the station possibly needs to switch to different channels in which the APs operate on. Second, the station needs to consume high power due to the long sessions of FTM frame exchange. Third, dense AP deployment is required to provide good coverage for supporting FTM positioning. Fourth, FTM traffic load increases when more stations perform positioning. The FTM protocol overhead can be substantial since all stations in a dense environment need to perform ranging independently.
In addition to the above-illustrated drawbacks of the FTM protocol, the TOF measurements with different responder APs at different time instances introduce uncertainties in the position solution. FIG. 1B (Prior Art) illustrates TOF measurements at different times. In the example of FIG. 1B, the dashed line depicts TOF measurements done at the same time instance, while the solid line depicts TOF measurements done at different time instances. It can be seen that unique position solution can be achieved only if all TOF measurements are performed at the same time instance. On the other hand, at different time instances t1, t2, t3, and t4, there is no unique position solution. Multiple sets of measurements are thus required for the position solution to converge, resulting in slow system response.
A solution for simplified Indoor Location operation with reduce airtime, lower STA power consumption, simpler operation, and easier deployment is sought.